The mammalian hematopoietic system comprises red and white blood cells. These cells are the mature cells that result from more primitive lineage-restricted cells. The cells of the hematopoietic system have been reviewed by Dexter and Spooncer in the Annual Review of Cell Biology 3, 423-441 (1987).
The red blood cells, or erythrocytes, result from primitive cells referred to by Dexter and Spooncer as erythroid burst-forming units (BFU-E). The immediate progeny of the erythroid burst-forming units are called erythroid colony-forming units (CFU-E).
The white blood cells contain the mature cells of the lymphoid and myeloid systems. The lymphoid cells include B lymphocytes and T lymphocytes. The B and T lymphocytes result from earlier progenitor cells referred to by Dexter and Spooncer as preT and preB cells.
The myeloid system comprises a number of cells including granulocytes, platelets, monocytes, macrophages, and megakaryocytes. The granulocytes are further divided into neutrophils, eosinophils, basophils and mast cells.
Each of the mature hematopoietic cells are specialized for specific functions. For example, erythrocytes are responsible for oxygen and carbon dioxide transport. T and B lymphocytes are responsible for cell-and antibody-mediated immune responses, respectively. Platelets are involved in blood clotting. Granulocytes and macrophages act generally as scavengers and accessory cells in the immune response against invading organisms and their by-products.
At the center of the hematopoietic system lie one or more totipotent hematopoietic stem cells, which undergo a series of differentiation steps leading to increasingly lineage-restricted progenitor cells. The more mature progenitor cells are restricted to producing one or two lineages. Some examples of lineage-restricted progenitor cells mentioned by Dexter and Spooncer include granulocyte/macrophage colony-forming cells (GM-CFC), megakaryocyte colony-forming cells (Meg-CFC), eosinophil colony-forming cells (Eos-CFC), and basophil colony-forming cells (Bas-CFC). Other examples of progenitor cells are discussed above.
The hematopoietic system functions by means of a precisely controlled production of, the various mature lineages. The totipotent stem cell possesses the ability both to self renew and to differentiate into committed progenitors for all hematopoietic lineages. These most primitive of hematopoietic cells are both necessary and sufficient for the complete and permanent hematopoietic reconstitution of a radiation-ablated hematopoietic system in mammals. The ability of stem cells to reconstitute the entire hematopoietic system is the basis of bone marrow transplant therapy.
It is known that growth factors play an important role in the development and operation of the mammalian hematopoietic system. The role of growth factors is complex, however, and not well understood at the present time. One reason for the uncertainty is that much of what is known about hematopoietic growth factors results from in vitro experiments. Such experiments do not necessarily reflect in vivo realities.
In addition, in vitro hematopoiesis can be established in the absence of added growth factors, provided that marrow stromal cells are added to the medium. The relationship between stromal cells and hematopoietic growth factors in vivo is not understood. Nevertheless, hematopoietic growth factors have been shown to be highly active in vivo.
From what is known about them, hematopoietic growth factors appear to exhibit a spectrum of activities. At one end of the spectrum are growth factors such as erythropoietin, which is believed to promote proliferation only of mature erythroid progenitor cells. In the middle of the spectrum are growth factors such as IL-3, which is believed to facilitate the growth and development of early stem cells as well as of numerous progenitor cells. Some examples of progenitor cells induced by IL-3 include those restricted to the granulocyte/macrophage, ecsinophil, megakaryocyte, erythroid and mast cell lineages.
At the other end of the spectrum is the hematopoietic growth factor that, along with the corresponding receptor, was discussed in a series of articles in the Oct. 5, 1990 edition of Cell. The receptor is the product of the W locus, c-kit, which is a member of the class of receptor protein tyrosine kinases. The ligand for c-kit, which is referred to by various names such as stem cell factor (SCF) and mast cell growth factor (MGF), is believed to be essential for the development of early hematopoietic stem cells and cells restricted to the erythroid and mast cell lineages in mice; see, for example, Copeland et al., Cell 63, 175-183 (1990).
It appears, therefore, that there are growth factors that exclusively affect mature cells. There also appear to be growth factors that affect both mature cells and stem cells. The growth factors that affect both types of cells may affect a small number or a large number of mature cells.
There further appears to be an inverse relationship between the ability of a growth factor to affect mature cells and the ability of the growth factor to affect stem cells. For example, the c-kit ligand, which stimulates a small number of mature cells, is believed to be more important in the renewal and development of stem cells then is IL-3, which is reported to stimulate proliferation of many mature cells (see above).
As mentioned above, c-kit is a protein tyrosine kinase (pTK). It is becoming increasingly apparent that the protein tyrosine kinases play an important role as cellular receptors for hematopoietic growth factors. Other receptor pTKs include the receptors of colony stimulating factor 1 (CSF-1) and PDGF.
The pTK family can be recognized by the presence of several conserved amino acid regions in the catalytic domain. These conserved regions are summarized by Hanks et al. in Science 241, 42-52 (1988), see FIG. 1 starting on page 46 and by Wilks in Proc. Natl. Acad. Sci. USA 86, 1603-1607 (1989), see FIG. 2 on page 1605.
Additional protein tyrosine kinases that represent hematopoietic growth factor receptors are needed in order more effectively to stimulate the self-renewal of the totipotent hematopoietic stem cell and to stimulate the development of all cells of the hematopoietic system both in vitro and in vivo. Novel hematopoietic growth factor receptors that are present only on primitive stem cells, but are not present on mature progenitor cells, are particularly desired. Ligands for the novel receptors are also desirable to act as hematopoietic growth factors. Nucleic acid sequences encoding the receptors and ligands are needed to produce recombinant receptors and ligands.